1. Field of the Invention
The present invention relates to a method for producing a food product containing a thrombolytic enzyme, nattokinase, but containing little or none of a blood coagulation factor, vitamin K2, in particular, a food product made from a culture of Bacillus natto. 
2. Description of the Prior Art
It was discovered by Sumi et al. that the Bacillus natto produces the thrombolytic enzyme nattokinase (Experientia Vol. 43, pp. 1110-1111 (1987)). The nutritional value of natto (fermented soybeans) and its value as a health food have been highly evaluated. It is known that nattokinase itself acts as a fibrinolytic enzyme. Nattokinase lyses thrombi when natto is ingested as a food, thus nattokinase has the thrombolytic activity. Nattokinase has extremely good characteristics such as having a long half life and retaining its effectiveness for a long period of time. In addition, it has been reported that nattokinase has a therapeutic effect on incipient central retinal vein occlusion although the effect cannot be clearly seen with urokinase, which also has the thrombolytic activity (Nishimura et al., Japanese Review of Clinical Ophthalmology Vol. 88, No. 9, pp. 53-57 (1994)).
Accordingly, food products containing large amounts of nattokinase, for example, powders and encapsulated products made from Bacillus natto culture have been marketed as health foods.
On the other hand, Bacillus natto is known to produce large amounts of vitamin K2. Vitamin K2 is known as an essential element in the blood coagulation system. Vitamin K2 has other physiological functions, and is said to cause absorption difficulties in newborns and osteoporosis in the elderly when it is deficient, and is said to cause disorders such as hemolytic anemia, splenomegaly, nephropathy, and hepatopathy when it is present in excess. Thus, a Bacillus natto culture extract contains not only nattokinase as an effector of thrombolytic system but also vitamin K2 as an effector of blood coagulation system.
The required daily intake of vitamin K's for adults is generally 55 to 65 μg. Large amounts of vitamin K's are contained in foods such as seaweed, broccoli and the like. Vitamin K's are produced by enterobacteria. Also, vitamin K's can be produced by Bacillus natto that grow in the intestines with ingesting natto. Therefore, it is said that the required intake of vitamin K2 can be satisfied with a normal diet. Thus, it is generally not necessary to supplement vitamin K2.
Further, problems regarding ingesting natto would occur with patients who receive inhibitors for the vitamin K-dependent blood coagulation factors such as prothrombins VII, IX, and X to prevent thrombosis. If the patients ingest a natto or a Bacillus natto culture extract, each of which contains the thrombolytic enzyme, nattokinase, for example, in order to prevent thrombosis, they also would intake vitamin K2 at the same time, and the effects of the inhibitors for the vitamin K-dependent blood coagulation factors would be counteracted.
Accordingly, in order to prevent thrombogenesis, it is desirable to obtain a food product made from Bacillus natto culture extract in which vitamin K2 has been reduced. Therefore, various methods have been attempted for reducing vitamin K2 in the food product. These methods include extraction of fat soluble vitamin K2 using organic solvents such as hexane. However, there have been some problems in this extraction method as described below. Fat soluble nutrients as well as vitamin K2 would be extracted out and reduced in the food product. The need to remove organic solvents would lead to an increase in the cost of manufacturing. The organic solvents would remain in the food products. The use of organic solvents would cause consumer to resist against ingestion.
A method for removing vitamin K2 using chitosan has been proposed (U.S. Pat. No. 6,730,504). However, chitosan must be dissolved in water before treatment, and is expensive. Therefore, there is a demand for alternative methods for removing vitamin K2.